Digital subscriber line (DSL) technology provides high-speed data transfer between two modems across ordinary telephone lines, wherein digital data transfer rates from tens of Kbps to tens of Mbps are supported over standard telephone lines (e.g., twisted pair of copper wires), while still providing for plain old telephone service (POTS). Asynchronous Digital Subscriber Line (ADSL) and Very High Digital Subscriber Line (VDSL) have emerged as popular implementations of DSL systems, where ADSL is defined by American National Standard Institute (ANSI) standard T1.413 and International Telecommunication Union (ITU-T) standards G.992.3, G.992.5, and VDSL is defined by ANSI standard T1.424 and ITU-T standard G.993.1. ADSL, VDSL and other similar DSL systems (collectively referred to as “xDSL”) typically provide digital data transfer in a frequency range above the POTS band (e.g., about 300 Hz to 4 kHz), for example ADSL G.992.3 operates at frequencies from about 25 kHz to about 1.1 MHz.
One feature of DSL modems that allows them to provide high data rates is their ability to communicate symbols over a multi-carrier channel 100, such as shown in FIG. 1. The illustrated multi-carrier channel 100 includes a number of frequencies or “carriers” 102 (e.g., carriers f1, f2, . . . fN) that span a frequency spectrum 104 supported by the telephone line. In effect, by breaking the frequency spectrum 104 into multiple carriers, the DSL modems can transmit data over each of the carriers (instead of just a single carrier), thereby allowing them to “stuff” more data through the phone line per unit time.
During communication, a specific number of bits per unit time may be transmitted on each carrier based on a signal-to-noise ratio (SNR) 106. Typically, more bits are transmitted on carriers that have a relatively high SNR, while fewer bits are transmitted on frequencies that have a relatively low SNR. For example, carrier f2 has a relatively high SNR compared to carrier f4. Correspondingly, the DSL modems transmit more bits on carrier f2 (i.e., approximately 18 bits per unit time), and fewer bits on carrier f4 (i.e., approximately 14 bits per unit time). Although encoding and decoding data conveyed on multiple frequencies makes this multi-carrier communication computationally complex, it gives the DSL modems the ability to provide users with high speed data connections with relatively few errors.
Although a multi-carrier channel affords DSL communication systems some ability to account for particularly noisy regions of the frequency spectrum, other components are typically used to account for more dynamic noise sources, such as impulse noise. Therefore, to achieve high data rates with high accuracy, improvements can be made to allow communication systems to communicate more efficiently.